NMR spectroscopy (NMR stands for nuclear magnetic resonance, nuclear spin resonance) is an instrumental analysis method, in which it is possible to determine, in particular, the chemical composition of samples. In this process, high-frequency pulses are irradiated into the sample, which is located in a strong, homogeneous static magnetic field B0, and the electromagnetic reaction of the sample is measured. In solid-state NMR spectroscopy, the sample is rotated about an axis of rotation at a high speed to reduce the line broadening caused by anisotropic interactions. To this end, the axis of rotation is tilted at the “magic angle” of θm=arccos(√⅓)≈54.74° to the homogenous static magnetic field. This measuring technique is commonly referred to as “magic angle spinning” (MAS). The angle θm is a solution to the second-order Legendre polynomial P2(cos(θm))=0, and so all interactions that depend upon this Legendre polynomial are averaged out over time during rotation about an axis at this angle to the magnetic field. This is the case for three important interactions in the substance: bipolar coupling, chemical shift anisotropy and first-order quadrupole interaction. Since the crystal directions of individual crystallites for non-single crystal samples are random relative to the static field, the interaction is cancelled out by the sufficiently fast rotation of the sample at the magic angle. In this way, line broadening caused by these interactions can be reduced significantly, ideally as far as the natural line width.
MAS-NMR measuring probes permit the use of high-resolution NMR spectroscopy on solid, powdery or semi-solid (gelatinous or pasty) sample substances. The sample is loaded into a circular cylindrical sample container, i.e. the rotor, which, during the measurement, is rotated using compressed gases in a stator to very high speeds at a rotation frequency in the range of a few KHz to over a hundred KHz. The radial bearing is secured by air bearings in the stator while a retaining force generated by air flow retains the rotor in its axial position in the stator.
The sample is surrounded by at least one transmitter and/or receiver coil, in which high-frequency (HF) alternating magnetic fields B1i can be irradiated into the measuring volume; the frequency ωi of said alternating fields is adapted to the resonance frequency of a core in the static magnetic field B0. Usually, measuring probes are designed such that at least two different frequencies can be transmitted and received.
Many solid-state NMR experiments utilize cross polarization (CP) to increase sensitivity. CP is a standard NMR technique for reinforcing and allocating NMR lines, in which rare spins S (e.g. spins of 13C nuclei) are coupled to common spins I (e.g. spins of 1H nuclei). In addition, two alternating magnetic fields B1I and B1S, each having the resonance frequency ωI and ωS of the respective spins, are irradiated onto the sample at the same time, where the two alternating magnetic fields also at least approximately satisfy the Hartmann-Hahn condition (HHC). The HHC is satisfied when the intensities of the spinlock fields B1I and B1S fulfill the condition γ1 B1I=γS B1S, γI and γS being the gyromagnetic ratios of the respective nuclear spins. An optimal polarization transfer is then possible between the two nucleus types I and S by J cross polarization. CP is a double-resonance experiment that is mostly carried out between 1H (I) and 13C (S).
In solids, the signal of S is strengthened by cross polarization in that I is retained for a contact period in the spinlock (spinlocking), S is brought to saturation and the “hot” and “cold” spins couple (spin temperature) such that a magnetization transfer occurs. In doing so, the common nucleus I is excited and its energy is transferred to the detected nuclear spin S by applying a low-power RF pulse to both channels. The HF power of these two pulses must be set such that the transition energy for both nuclei is the same. For the example of the 1H 13C polarization transfer, the high-frequency magnetic field B1 must be of the order of four times weaker for the proton channel than for the carbon channel. The efficiency of the polarization transfer is highly dependent upon satisfying the Hartmann-Hahn condition. In particular, it is important that the Hartmann-Hahn condition be satisfied at as many points as possible in the measuring volume within a tolerance range, which depends upon various experimental parameters, inter alia the MAS rotation frequency.
One experiment with an especially narrow tolerance range for fulfilling the HHC, which holds great significance in the solid-state NMR of biological substances, is the double CP experiment (DCP). In this experiment, two CP steps are carried out in succession. Usually, the polarization of the protons is transferred to an X nucleus in a first step (to increase sensitivity), and transferred to a different (Y) nucleus in a second step in order to determine the dipolar coupling between X and Y nuclei. The sequence of the transfer is arbitrary in principle, but it normally proceeds from 1H to 15N and finally to 13C, which is ultimately detected.
The tolerance ranges of the HHC for this experiment are extremely tight, and so it is necessary to perform an exact calibration of the B1i field strengths. Furthermore, the efficiency is extremely dependent upon all three B1i fields involved interacting appropriately, in particular the alternating magnetic fields for exciting the 15N and 13C nuclei in the measuring volume.
Generally, NMR measuring probes are employed in superconducting NMR magnet systems, in which the homogeneous static magnetic field B0, which defines the z axis of the laboratory coordinate system, is oriented along a “bore”.
The samples to be analyzed, consisting of liquid, powdery, gelatinous substances, but also tissue samples, single crystals, glasses or mixtures of different substances, are usually loaded into cylindrical, in particular circular cylindrical, containers. This sample container is inserted into a recess in the measuring probe provided for receiving samples. The cylinder axis of the sample container is usually either parallel, orthogonal or at the magic angle θm=arccos(√⅓)≈54.74° to B0. The samples are surrounded by generally cylindrical RF coil assemblies, which usually consist of saddle coils, solenoid coils and/or resonators, birdcage resonators and Alderman-Grant resonators in particular being used.
It is conventional during an NMR experiment to employ the same coil to excite the nuclear spins, i.e. as a transmitter coil, and to detect signals in a later phase of the NMR experiment, i.e. as a receiver coil. In so doing, the corresponding HF paths of the NMR spectrometer are correspondingly switched from an HF transmitter to a preamplifier and HF receiver. Corresponding switching devices are known.
Based on the extremely small signals that are measured during NMR experiments, the sensitivity in the detection phase often has an especially high importance during the design of a coil assembly. An NMR measuring probe usually has one and optionally two preferred measuring frequencies that are used for detection. Further measuring frequencies are utilized primarily for transmitting decoupling pulses and for polarization transfers. In order to permit multiple transmission or reception frequencies to be operated simultaneously, two coil assemblies that lie radially inside each other are generally used. At least one of the detection measurement frequencies is applied to the inner coil assembly, and an attempt is made to minimize the losses of these coil assemblies. This results in the optimization of the sensitivity of the measuring probe during detection.
Measuring probes in which multiple resonance frequencies are tuned on a single detection coil are conventional particularly in solid-state NMR, although this results in significant losses in performance when more than two resonance frequencies are involved. Satisfying a number of boundary conditions leads to significant losses in the performance of CP experiments, in particular of DCP experiments.
An NMR measuring probe is known from U.S. Pat. No. 6,806,713 B2, which employs as many as three coils, which are arranged coaxially with a MAS axis of rotation, at least two of said coils being tuned to the same measurement frequency and operated in quadrature. A measuring probe of this type minimizes the power required to generate broad-band RF pulses and maximizes sensitivity for the detection of the measuring channel operated in quadrature. The coil assembly in this measuring probe solves the problem that only the components of the alternating magnetic fields standing perpendicularly to the static magnetic field are effective for the excitation of the nuclear spins and that the alternating magnetic fields to be received are likewise perpendicular to the static magnetic field. The coils according to U.S. Pat. No. 6,806,713 B2 are interconnected and operated such that the alternating magnetic fields generated thereby add up to an alternating magnetic field with a large active component, and/or said coils optimally detect the alternating magnetic fields generated by the nuclear spins in the receiving phase of an NMR experiment.
The problem of cross polarization efficiency remains unsolved in the aforementioned coil assembly, in particular the problem of cross polarization efficiency in DCP experiments. For this reason, a solution such as the one presented in U.S. Pat. No. 6,806,713 B2 does not constitute a maximization of the measuring probe sensitivity for realistic experiments.
An alternative HF coil assembly for generating at least two independent alternating magnetic fields in an examination volume of a magnetic resonance apparatus which overcomes a disadvantage associated with the prior art is provided herein. In particular, an HF coil assembly having high cross polarization efficiency (CP efficiency) is provided herein.